Ccna routing and Switching Complete Study Guide

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Urgent    A valid field only if the Urgent pointer in the code bits is set. If so, this value 

indicates the offset from the current sequence number, in octets, where the segment of non-

urgent data begins.

Options    May be 0, meaning that no options have to be present, or a multiple of 32 bits. 

However, if any options are used that do not cause the option field to total a multiple of 

32 bits, padding of 0s must be used to make sure the data begins on a 32-bit boundary. 

These boundaries are known as words.

Data    Handed down to the TCP protocol at the Transport layer, which includes the upper-

layer headers.

Let’s take a look at a TCP segment copied from a network analyzer:

TCP - Transport Control Protocol

 Source Port:      5973

 Destination Port: 23

 Sequence Number:  1456389907

 Ack Number:       1242056456

 Offset:           5

 Reserved:         %000000

 Code:             %011000

      

Ack is valid

      

Push Request

 Window:           61320

 Checksum:         0x61a6

 Urgent Pointer:   0

 No TCP Options

 TCP Data Area:

 vL.5.+.5.+.5.+.5  76 4c 19 35 11 2b 19 35 11 2b 19 35 11

  2b 19 35 +. 11 2b 19

Frame Check Sequence: 0x0d00000f

Did you notice that everything I talked about earlier is in the segment? As you can see 

from the number of fields in the header, TCP creates a lot of overhead. Again, this is why 

application developers may opt for efficiency over reliability to save overhead and go with 

UDP instead. It’s also defined at the Transport layer as an alternative to TCP.